Detector circuit



`Iuly 28, 1936.

P. O. FARNHAM DETECTOR CIRCUIT Filed Oct. 14, 1931 PAUL o. FA HAM BY fg ,Lav/W ATTORNEY Patented July 28, 191i@ DETECTOR CIRCUIT Delaware Application October 14, 1931, Serial No. 568,674

22 Claims.

My present invention relates to detector circuits, and more particularly to acombined amplifying detector and diode rectifier circuit especially adapted for use in connection with radio receivers employing automatic gain control.

It has been recognized in the past that one of the cold electrodes of a detector tube could be employed in conjunction with the electron emission electrode to provide a diode rectifier which could be employed for automatic gain control of the radio frequency amplifier tubes of a radio receiver. It has been well understood in the prior art that detector circuits could be arranged to handle large amounts of radio frequency power, and accordingly produce large audio frequency outputs, by employing a type of detection known as grid bias detection, or anode rectification, as opposed to the well known type of grid leak detection. Such a. detector circuit has been termed a power detector stage, Vand has usually been so designed as to require only a single Stage -of audio frequency amplication for energizing a loud speaker of a radio receiver.

This invention provides a circuit arrangement which consists essentially of the combination of a triode power detector and a diode rectifier within the same vacuum tube. A carrier frequency input obtained from a carrier frequency amplifier is applied to these two units in parallel. The useful output of the triode power detector is obtained as a voltage of the modulation frequency appearing across an audio frequency impedance connected in the circuit between screen grid and cathode. This audio voltage may then be further amplified up to the speaker. The useful output of the diode rectifier is obtained as a direct current potential drop across a resistor connected in the circuit between anode and cathode. This potential drop is then used to control automatically the gain of the carrier frequency amplifier by increasing its control grid bias in the well known manner.

Accordingly, it may be stated that it is one of the main objects of my present invention to provide a circuit for detecting high frequency energy, which circuit includes an electron discharge tube provided with an electron emission electrode, a control grid, an anode Vand a screen grid, the circuit being so arranged that the cathode,l control grid and screen grid cooperate to function as a triode detector circuit operating on the anode rectification principle, while the anode of the said tube functions with the electron emission electrode as a diode rectifier to auto- (Cl. Z50-27) matically control the gain of preceding high frequency amplifier stages.

Another important object of the present invention is to provide a detector circuit, in a radio receiver, utilizing a screen grid tube, which detector circuit is arranged to function as a power n detector, and simultaneously therewith, whose output circuit includes a device, cooperating with the cathode of the tube and at least one of the cold electrodes of the tube, to control the gain of the radio frequency amplifier stages of the receiver.

Still another objectV of the present invention is to provide a `detector circuit employing a screen grid tube wherein the screen grid is maintained at a higher direct current potential than the anode of the tube, and the control grid is maintained at a lower radio frequency potential than the said anode, the cathode and anode of the tube being utilized to function as a diode rectifier in an automatic gain control system.

Still another object of the present invention is to provide in a radio receiver a detector circuit utilizing a screen grid tube, means for coupling the output of the tube to a low frequency amplifier, the circuit being arranged to rectify high frequency energy by anode rectification whereby the detector circuit functions as a power detector.

And still other objects of the present invention are to improve generally the simplicity and efficiency of radio receivers employing -automatic gain control circuits, and to especially provide a detector circuit in such -a receiver which is not only reliable in operation, but is capable of -handling large amounts of radio frequency power.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically one circuit arrangement whereby my invention may be carried into effect.

Referring to the accompanying drawing there is shown a high frequency receiving system, as for exampleV a radio receiver, in conventional manner. At A there is represented a multi-stage radio frequency amplifier, it being clearly understood that the representation designates any amplifier well known to those skilled in the art, and which customarily employs a plurality of tuned radio frequency amplifier stages, each stage utilizing an electron discharge tube.4 The source of radio frequency energy, usually coupled to the input of the amplifier A, is not shown, and it will be understood that the source may be, for example, an antenna system. The output of the amplifier A is coupled, as at M, to the input electrodes of a detector tube I.

The latter is preferably an electron discharge tube of the well known 224 type. That is to say, the tube embodies a cathode for emitting electrons, a control grid, an anode, and an auxiliary screen grid. The detector circuit, as has been heretofore pointed out, is arranged in a manner not known to those skilled in the prior art, the arrangement being such that the detector stage is capable of handling large amounts of radio frequency power, and simultaneously able to control the gain of the amplifier A in an automatic manner.

To secure the aforementioned novel results and functions, the tunable oscillatory circuit, comprising the coilv L and the variable condenser C, is connected between the anode Zand the cathode 3 of the tube I. The anode 2 is connected to the high potential side of the condenser C through a fixed capacity 4 having a high impedance for the modulation frequency,and having an impedance for the carrier frequency low compared to the anode-cathode impedance of the tube, while the cathode 3 is connected to the low potential side of the condenser C vthrough the Vbiasing resistor 5, which has connected in shunt therewith the fixed by-pass capacity 6. Asis well known to those skilled in the art the resistor 5 and condenser 6 provideV an arrangement for biasing the control grid 'I of the detector tube.

The control grid 'I is preferably connected to a point on the coil L of a lower potential than the point to which the anode 2 is connected, and for purposesof convenience in adjustment the control grid connection to the coil L is made adjustable. The reason `for connecting the grid .'I and the anodel2, as shown, Vwill be hereinafter stated in detail. The output circuit of the detector further includes the screen grid electrode 8 which is connected in series with the positive terminal of a source of uni-directional current B through the primary coil 9 of an audio frequency transformer III. The negative terminal of the source B is connected by a lead I I to the low potential sides of the condenser 6 and the resistor 5. It is to be clearly understood that the source B need not be a direct current battery as shown on the drawing, but may be. any well known source of unidirectional current,.such as for example, the output terminals of a filter system employed for filtering rectified alternating current.

rThe secondary I2 of the transformer I0 may be connected to any desired type of utilization means,

such as a loud speaker, since the output circuit of the tube I is capable of delivering a large audio frequency output, in fact sufficiently large to operate directly a loud speaker. However, there maybe employed, between the screen `grid circuit and the utilization means, a stage of audio frequency amplification conventionally represented as at D. In View of the fact that the detector circuit is of the power type, a single stage of audio frequency amplification will be suicient to operate a loud speaker for all general purposes 'in reproducing broadcast programs. It will, thus,

be seen that the cathode 3, control grid 1, and screen grid 8 cooperate to function as a triode detector employing anode rectification and additionally that the screen grid electrode functions Of course the variable condenser C may be arranged for simultaneous and similar operation with the variable condensers usually employed in the tuned stages of the radio frequency amplifier A.

It has been stated, heretofore, that one of the 5 prime objects of the present invention is to provide a detector stage not only capable of functioning as a power detector, but simultaneously adapted to operate as the control mechanism in an automatic gain control system for the radio frequency amplifier. This object is realized by arranging the anode circuit of the detector tube I in such a manner that the anode 2 functions in conjunction with the cathode 3 as a diode rectier to produce a direct current potential drop across an impedance arranged between the anode and the cathode, which drop may be utilized to bias the preceding stages of the amplifier A.

More specifically, the said impedance comprises a resistor R connected between the anode 2 and a desiredV point on the grid bias resistor 5, the connection to the latter preferably being made adjustable. The potential drop across the resistor R is employed for biasing the radio frequency amplifier grid circuits by means of a lead I3 having one end thereof connected to the low potential side of the resistance R as at I 4, and its opposite end connected to each of the grid circuits of the radio frequency amplifier.

The latter connection to the grid circuits has 30 not been shown in detail for the reason that such a connection is so well known to those skilled in the art that the present description is believed sufficient to indicate the manner in which the gain control arrangement is to be constructed. The lead I3 includes in series therewith a resistor I5, a fixed capacity I6 being connected between the negative side of the resistor I5 and the negative side of the resistor 5. As is well known to those skilled in the art the capacity I6 and resistor I5 function as a filter arrangement for the current owing through the conductor I3.

The operation of the circuit is believed clear from the aforegoing description, it being, additionally, pointed out that the amplified received energy is impressed across the electrodes I and 3, the detected energy being taken off the screen grid circuit, anode rectification being employed, and the detected energy being amplified and utilized. With regard to the action of the anode 2 and cathode 3 as a diode rectifier, it will be noted that the anode 2 does not have any steady positive potential applied to it in the customary manner, but is rather maintained at a negative biasy with respect to the cathode 3 by means of tap II.

It will now be understood that during the positive halves of the received carrier swings, electrons'from the cathode flow towards the anode and through the resistor R, thereby rendering the point I4 more negative than the opposite end of the resistor R. By virtue of the connection of the lead I3 to the negative end of the resistor R, it is possible to employ the variations of potential drop across the resistor R to bias the grid circuits of the preceding amplifier stages in such a manner as to maintain a substantially uniform radio frequency voltage applied to the detector for a wide range of signal levels at the amplifierinput. It has been found extremely desirable tol make the direct current drop across the resistor R start at a carrier level well below that required to overload the control grid l. This result may be obtained by either or both of two methods.' By adjusting the direct current 75 bias-'between anode 2- and cathode v'through the position of tap l l toa value lower than the direct: current bias between Athe control grid 7 and the.'y cathode 3,' an increasing carrier voltage applied--V to the systemy will cause useful biasing current .to flow in resistor R before the control .grid lcircuitV overloads. It will also=beseen that bil-adjusting', the connection of the control grid T to alpoint on the supply circuit L, C, of lower carrier-frequency potential than that applied to anode 2, an increasing carrier voltage applied to the system willv cause current toflow in resistor R before .the con-- trol grid circuit overloads, even -though tap I1- were brought to the lower end of cathode -biasi While I have indicated and described one arrangement for Ycarrying .my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organization shown and described but that many modifications may be made withoutdeparting from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In combination, a radio frequency amplifier,v a detector stage including an electron discharge tube provided with a cathode, control grid, screen: grid and anode, a tunable oscillatory circuitcon-r nected between the control grid and the cathode and to said anode, means in the cathode circuit for biasing the control grid negative with respect to the cathode, means for maintaining thescreen grid positive with respect to the cathode, means for maintaining the anode at an average potential less positive than thecathode, and means in cluding an impedance connected between the anode and cathode, for automatically controlling the gain of said amplifier.

2. In combination, a radio frequency amplifier', a detector stage including Aan electron discharge tube provided with a cathode, control grid, screen grid and anode, a tunable oscillatory-circuit connected between the control grid and the cathode, means in the cathode circuit for biasing the control grid negative with respect to the cathode,

means for maintaining the screen grid positive with respect to the cathode, means for maintaining the anode at an average potential less positive than the cathode, and means including an impedance connected between the-anode and` cathode, for automatically controlling the gain of said amplifier, said anode being connected 'to a high potential point of said oscillatoryl circuit.A

3. In combination, a radio frequency amplifier, a detector stage including an electron discharge tube provided with a cathode, controlgrid, screen grid and anode, a tunable oscillatory circuit con-f nected between the control grid and the cathode, means in the cathode circuit for biasing the control grid negative with respect to the cathode, means for maintaining the screen gridk positiveV with respect to the cathode, means for maintaining the anode at an average potential less positive.

than the cathode, and meansincluding an impedance connected between the anode` and cathode, for automatically controlling the gain of said amplier, said anode being connectedfto a point in said oscillatory circuit 4'of higher radio frequency potential than the point to which the control grid is connected.

4. In combination, a radio frequency amplifier,v a detector stage including an electron discharge tube provided with a cathode, control grid, screen grid and anode, a tunable oscillatory circuit connected between the control gridand the cathode and to'theanode, means in the lcathode circuit -for biasing. the control grid negative with respect to the cathode, means for maintaining the screenl tial less ;positive than the cathode, and means including an impedance connected between thev anode and cathodafor automatically controlling the gain of said amplifier, and at least one stage of audio frequency amplification coupled to the screen grid circuit.

5; In combination, a radio frequency amplifier, a Adetector stage including an electron discharge tube provided withV a cathode, control grid, screen grid and anode, a tunable oscillatory circuit connected between the control grid and the cathode and to the anode, means in the cathode circuit for biasing the control grid negative with respect tothe cathode, means for maintaining the screen grid positive with respect to the cathode, means for maintaining the anode at an average potential less positive than the cathode, and means including an impedance connected between the anode and cathode, for automatically controlling the` gain of said amplier, said impedance comprising a resistor connected between the anode and the control grid biasing means.

6. In combination, a radio frequency amplifier, adetector stage including an electron discharge tube provided with a cathode, control grid, screen grid and anode, a tunable oscillatory circuit connected between the control grid ,and the cathode., means in the cathode circuit for biasing the control grid negative with respect to the cathode,l

means for maintaining the screen grid positive with respect to the cathode, means for maintainthe high potential side of said oscillatory circuit,

and said control grid being adjustably connected to a point on said oscillatory circuit of a lesser radio frequency potential than the point to which the anode is connected, said impedance comprisingv a resistor connected between the anode and said control grid biasing means.

7. A frequency changing circuit comprising a space discharge device provided with a cathode, control grid, screen grid and anode, an oscillatory high frequency circuit connected between said anode and cathode, high frequency current transmission means for connecting said control grid to a point of said oscillatory circuit of lesser high frequency potential than the point to which the said anode is connected, means for negatively biasing said control grid, means for positively biasing said screen grid with respect to said anode and cathode, and a resistive path connected between the anode and cathode whereby the anode and cathode function as a diode rectifier.

8; A frequency ychanging circuit comprising a space 'discharge device provided with a cathode, control grid, scr-een grid and anode, an oscillatory lcircuit connected between said anode and cathode, means for connecting said control grid to a point of said oscillatory circuit, means for negatively biasing said control grid, means for positively biasing said screen grid with respect to said anode and cathode, and a resistive path connected between the anode and cathode whereby the anode and cathode function as a diode rectier, a high frequency amplifier coupled to said oscillatory'circuit, and means between the ampotential drop across said path to control theV gain of said amplifier.

9. A frequency changing circuit comprising a.

space discharge device provided with a cathode, control grid, screen grid and anode, an oscillatory circuit connected between said anode andY cathode, means for connecting said control grid to a point of said oscillatory circuit, means for negatively biasing said control grid, means for positively biasing said screen grid with respect to said anode and cathode, and a resistive path connected between the anode and cathode whereby the anode and cathode function as a diode rectifier, a high gain radio frequency amplifier coupled to said oscillatory circuit, at least one stage of audio frequency amplification coupled to the screen grid circuit, and means connected between said amplifier and said resistive path for employing the potential drop across said path to control the gain of said amplifier.

10. In combination, a radio frequency amplier, a detector stage including an electron discharge tube provided with a cathode, control grid, screen grid, and anode, means for impressing radio frequency amplifier output voltage between the control grid and cathode, means for biasing the control grid negative to the cathode, means for maintaining the screen grid positive with respect to the cathode, means for impressing radio frequency amplifier output voltage between anode and cathode, means for maintaining the average anode potential less positive than the cathode, and means, including a resistance connected between anocle and cathode, for furnishing a rectified potential for automatically controlling the gain of said amplifier.

1l. In combination with a radio frequency amplifier of the type including at least one tube provided with a gain control electrode, a stage following the amplifier including a tube provided with a cathode and at least three cold electrodes, an audio frequency output circuit connected between one of the cold electrodes and said cathode,

- a tuned radio frequency input circuit connected between a second of said cold electrodes and said cathode, the third cold electrode being connected to a point on said input circuit of lesser'radio frequency potential than the point to which the second cold electrode is connected, a path, including an impedance, connected between the second cold electrode and the cathode to provide a gain control bias for the said ampliler tube, and a direct current connection between the path and f a the gain control electrode of the amplifier tube.

12. In combination with a high frequency amplifier tube provided with a gain control electrode,

a detector tube including a cathode, a grid, aA

cold positive electrode, and an auxiliary cold electrode, a tunable oscillation circuit connected be-A tween the auxiliary electrode and the cathode,

an audio frequency output circuit connected be 'l means for biasing the grid to render the cathode,

grid and positive electrode operative as an anode detection circuit, means for connecting the auxiliary electrode and cathode to provide a diode rectifier, and a direct current connection between the auxiliary electrode and said gain control electrode of the amplifier tube.

13. In combination with a radio frequency signal amplier of the type including at least one tube provided with a gain control electrode, a detector stage following the amplifier and incldig'a tub`e provided with a cathode and at least three cold electrodes, an audio frequency output circuit connected between one of the cold electrodes and said cathode, a signal input network connected between a second one of the cold electrodes and said cathode, the third of said cold electrodes being connected to said input network, both said second and third cold electrodes being connected to 'the high alternating current potential side of said network, a resistor in circuit with at least said secondcold electrode and said cathode for'developing a direct current gain control bias for said amplifier tube, and a direct current connection between a point of negative potential on said resistor and the said gain control electrode. Y

14. In a radio receiver utilizing a single heated source of electron emission for a plurality of electron paths, the method of reproducing radio frequency signals into perceptible condition at uniform volume which comprises, amplifying said signals atI a frequency above audibility, translating said amplified signals into audio frequency signals with a substantial translation gain thrughone of said electron paths, rectifying a component of said amplified signals through anotherfof said paths', utilizing said rectified signal component in regulating the amplification of the signals 'above audibility, and translating said audio frequency signals into perceptible condition. A Y

15. In a'radio receiver utilizing a single heated source of electron emission for a plurality of electron paths,the method of reproducing radio frequency signals into perceptible condition at uniform volume which comprises, amplifying said signals -at a frequency above audibility, translating said amplified signals into audio frequency signals with a substantial vtranslation gain through one of said electron paths, rectifying a component of said amplified signals through another of said paths, filtering said' rectified component to eliminate a substantial pulsating component therefrom, developing from said rectified and filtered component a regulating potential, utilizing said potential to regulate the amplification of signals` above audibility, andl translating said audio frequency signals into audible signals.

16. In a radio receiver including an amplifier having means to regulate the degree of amplification to be effected thereindetermined by a biasing potential, and a detector of the electron discharge type utilizing'a single heated source of electron emission for a plurality of electron paths, the method Aof reproducing radio frequency signals into perceptible condition at uniform volume which comprises, amplifying signals at a frequency above audibility, utilizing one electron pathto translate said amplified signals into audio frequency signals and to amplify the same, also utilizing another electron path to rectify the carrier wave component of the first-mentioned amplified signals, employing said rectified carrier wave component to develop a biasing potential, applying said biasing potential to regulate the signal amplification effected above audible frequencies, and translating said audio frequency signals into perceptible condition.

L 17.' In a radio receiver including an amplifier provided with means to regulate the degree of amplification to be effected therein as determined by a biasing potential, and a detector o-f the electron discharge type utilizing a single heated source of` electron emission for a plurality of electron paths', the methodiof reproducing radio frequency signals into perceptible condition at uniform volume which comprises, amplifying signals at a frequency above audibility, utilizing one electron path to translate said amplified signals into audio frequency signals With an appreciable translation gain and utilizing another electron path to rectify the carrier Wave component of the amplified signal, employing said rectified carrier wave component to develop a biasing potential, and applying said biasing potential to said amplifier to regulate the amplification effected therein, and translating said audio frequency signals into perceptible condition.

18. In a radio receiver including an amplifier for amplifying radiofrequency signals, said amplifier having means to regulate the degree of amplification effected therein as determined by a biasing potential, and a detector of the thermionic type utilizing a single heated source of electron emission for a plurality of electron paths, the method of reproducing radio frequency signals into perceptible condition at uniform volume which comprises, amplifying said signals at a frequency above audibility, developing electron emission from said source, utilizing a portion of said electron emission in one path to translate said amplified signal into audio frequency signals and to amplify the same, utilizing another portion of said electron emission in another path to develop a rectified current corresponding to the carrier wave component of said first-mentioned amplied signals, employing said rectified current to generate a biasing potential, utilizing said biasing potential to regulate the amplification of said first-mentioned signals, and translating said audio frequency signals into perceptible condition.

19. In a radio receiver, an amplifying stage including a vacuum tube operating to amplify signals at frequencies above audibility, a detector having a vacuum tube provided with a first electrode, a second electrode, a third electrode, and a cathode, anv input circuit for said detector linked to said amplifier, said input circuit including said cathode and one of said electrodes, an output circuit including said cathode and another of said electrodes, a rectifier circuit including a portion of said input circuit, an additional one of said electrodes and said cathode, said rectifier circuit serving to rectify the carrier component of said amplified signal, means for developing a potential according to said rectified carrier component, and means for utilizing said potential to control the amplification effected in said amplifying stage.

20. In a radio frequency signaling system, means for amplifying signals at frequencies above audibility, means including one heated electron source for translating said amplified signals into audio frequency signals with a substantial translation gain, means including said heated electron source for rectifying a component of said amplified signals, means controlled by said rectified signal component and including a portion of said amplifying means for regulating the amplification effected by said amplifying means, and means for reproducing said audio frequency signals into perceptible form.

21. In a radio frequency signaling system, means for amplifying signals at frequencies above audibility, means including one heated electron source for translating said amplified signals into audio frequency signals with a substantial translation gain, means including said heated electron source for rectifying a component of said amplified signals, means for filtering said rectified signal component, means controlled by said filtered signal component and including a portion of said amplifying means for regulating the amplification effected by said amplifying means, and means for reproducing said audio frequency signals into perceptible form.

22. In a radio frequency signaling system, means for amplifying signals at frequencies above audibility under the control of a biasing potential, means including one heated electron source for translating said amplified signals into audio frequency signals with a substantial translation gain, means including said heated electron source for rectifying a component of said amplified signals, means controlled by said rectified signal component for controlling the biasing of said amplifying means whereby the amplification effected by Said amplifying means is regulated, and means for reproducing said audio frequency signals into perceptible form.

PAUL O. FARNHAM. 

